There's tremendous variability in the physical geology of southern Utah, and for that reason, great variability in the life-zones and sub-ecosystems found in deserts, foothills, canyons, cliff-tops and mountains.
A life zone can be defined as any of a series of biogeographic zones into which a continent, region, etc. is divided both by latitude and altitude on the basis of the characteristic animal and plant life in a zone. At a largest scale, this can be seen in the life zones or biomes ranging from the tropics to the poles. On a smaller scale, it can be seen on the rising slopes of a mountain.
And, as Michael Plyler of Zion Field Institute has noted, then there's the presence or absence of water, and how that drives erosive forces and grows plant life.
The low point in southwest Utah is Beaverdam Wash, near St. George in Washington County, at 2,000 feet. Considering that the mean altitude for the rest of Utah is 6,092 feet, everything is looking up from Beaverdam Wash, including Zion National Park and neighboring country.
Elevations in Zion National Park range from the lowest at Coalpits Wash in the southwest corner (3,666 feet) to Horse Ranch Mountain in the Kolob Canyons area (8,726 feet). The rim of neighboring Bryce Canyon National Park varies from 8,000 to 9,000-feet, while Cedar Breaks National Monument -- a giant ampitheater -- is over 10,000 feet in altitude and as much as 2,000-feet deep.
Dixie National Forest, which borders Zion, Bryce Canyon, Cedar Breaks and Grand Staircase-Escalante National Monument, is mountainous country. Within a half-day's drive, you can venture from the alpine meadows and forests around Boulder to the edge of the Mohave Desert outside of St. George.
Since timberline starts at 11,500 feet, there's really no place you can go to find a timberline life zone in southwestern Utah.
The upper montane life zone is between 8,000 and 9,000 feet and is characterized by Englemann spruce, white fir, Limber pine, Bristlecone pine, Douglas fir, Quaking aspen and Subalpine fir.
The lower montane life zone occurs between about 6,200 and 9,500 feet above seal level. Most of the tree species are similar to those found at higher elevations, but aspen groves are more extensive. Willows and river birch can be found streamside.
The foothills life zone is between 5,000 feet and 6,500 feet elevation. Here you'll find box elder, cottonwood, hawthorn, buffalo berry, sumac, poison ivy, rose, common juniper, cliffrose, serviceberry, sagebrush, pinyon pine, juniper, ephedra (Mormon tea) and the sego lily.
The Northern Desert life zone typifies the eastern Great Basin between about 4,000 feet and 5,000 feet elevation. Here is big sagebrush, black sagebrush, shadscale or saltbrush, matchweed or snakeweed, winterfat, hopsage and mat saltbush.
Down south of St. George is the Southern Desert life zone, which extends on into Mexico. Here you can find sand sagebrush, sunflowers, creosote bush, yucca, Joshua tree and various cacti like cholla and prickly-pear.